Advanced Research About High-temperature And High-rateperformance Of The Nickel Hydroxide Material For Ni-MH Power Sources

In the 21 st century, the biggest challenge that human society facing is the worsening of ecological environment and the exhaustion of energy caused by over-exploitation of oil, coal and other non-renewable resources. Nowadays, with the rising popularity of all kinds of transportations, human beings require much more amount of energy. Face to the problem of energy and environment, the new chemical power source is playing a more and more important role. Actually, Ni-MH battery, as one of the new chemical power sources, is attracting widespread attention because of its advantage of environmentally friendly, high specific energy, high safety, good cyclability and low cost.To satisfy the requirement of Ni-MH battery used as power battery for electric vehicles(EVs) and hybrid electric vehicles(HEVs), extensive efforts have been dedicated to developing the high-temperature and high-rate power performance of Ni-MH batteries. As we know, the nickel hydroxide electrode which is the positive electrode of Ni-MH battery, determining the whole cell capacity, is the key factor that promising the good performace of the cell, so improving the electrochemical behaviour of nickel hydroxie is an effective way to accelerate the application of Ni-MH batteries. On the base of the commercial spherical β-Ni(OH)2, three aspects of works have been done in order to improve the high-temperature and high-rate performance of the nickel hydroxie electrode, the results are as following:(1) By using Na OH electrolyte containing Na BO2 additive, the high-temperature charge-discharge performance of sealed Ni-MH battries is successfully improved. Compared with conventional batteries using KOH electrolyte, the in-house prepared batteries exhibit an enhanced discharge capacity, improved high-rate discharge ability and increased cycle stability at an elevated temperature(70 ℃). The charge acceptance of the Ni-MH battery at 70 ℃ is over 96% at 1 C charge/discharge rate. CV tests indicate that the charge acceptance of Ni-MH battery is improved due to the high overpotential for oxygen evolution. By comparison, the smaller Rct of nickel electrodes which using the electrolyte with the introduction of Na BO2 reveal that the additive of Na BO2 can effectively reduce the charge-transfer resistance. The result of the self-discharge tests shows that the charge retention of the MH-Ni batteries at high-temperature can be notably improved through the use of Na OH electrolyte with Na BO2 additive. It is believed that this distinctive approach is promising and effective to improve the high-temperature electrochemical performance of alkaline rechargeable Ni-MH batteries.(2) γ-Co OOH which possesses high electrical conductivity was successfully coated onto spherical Ni(OH)2 through surface modification, the high-temperature and high-rate performances of nickel hydroxide electrodes in nickel-metal hydride(Ni/MH) batteries are improved significantly. Compared with conventional cells using uncoated Ni(OH)2, the in-house prepared cells using coated Ni(OH)2 electrode without adding any conductive additives exhibit an enhanced discharge capacity, improved high-rate discharge ability and better cycle stability at room temperature and an elevated temperature(70 ℃). CV and steady-state polarization tests indicate that γ-Co OOH coating can efficiently enhance the oxygen evolution overpotential and depress the oxygen evolution rate, leading to a higher charge acceptance. EIS studies show that with coated γ-Co OOH, the conductivity of spherical Ni(OH)2 is highly improved and the charge-transfer resistance is significantly decreased. The increased oxygen evolution potential and the decreased charge transfer resistance are attributed to the high conductivity coating of γ-Co OOH.(3) The effect of adding nanoscale-Ca(OH)2 on the high-temperature performance of nickel hydroxide electrode has been discussed. Following the coating of spherical β-Ni(OH)2 by γ-Co OOH, innovatively, nanoscale-Ca(OH)2 was introduced as a positive additive which could disperse uniformly on the surface of active materials of the electrode. Actually, there may be natural synergistic effect between the γ-Co OOH coating and nanoscale-Ca(OH)2 additive. After the doping of nanoscale-Ca(OH)2, the oxygen evolution potential and the charge acceptance are both improved, therefore, the nickel hydroxide electrodes with Ca(OH)2 additives can exhibit much better high-temperature electrochemical properties.